ADP released from activated platelets recruits nearby platelets, resulting in explosive accretion of a thrombotic nidus. ATP, released from activated platelets or injured cells, promotes inflammation. The coordinated phosphohydrolysis of extracellular nucleotides, by the sequential actions of the ectonucleotidases CD39 (nucleotide diphosphohydrolase 1, converts ATP-> ADP->AMP) and CD73 (51 nucleotidase, converts AMP->adenosine), is an important endothelial homeostatic mechanism which limits thrombosis and inflammation at the blood-vessel interface. CD39 gene null mice exhibit a latent prothrombotic phenotype and worse outcomes than controls in the setting of focal cerebral ischemia. These mice can be rescued by recombinant soluble CD39, which retains apyrase activity, without increasing intracerebral hemorrhage. Furthermore, hypercholesterolemic ApoE/CD39 double knockout mice exhibit exaggerated atherogenesis, consistent with a role for platelet and inflammatory cell recruitment into the developing plaque. These data suggest that ectonucleotidases protect against atherothrombotic events and are relevant to the pathoaenesis of stroke. Experiments will elucidate mechanisms by which CD39 and CD73 are atheroprotective, focusing on their ability to suppress platelet activation and inflammatory cascades, using wild-type, cd39- or cd73-gene null mice in control or hypercholesterolemic backgrounds. Experiments will test whether ectonucleotidases improve ischemic stroke outcomes in an atherosclerosis-prone cerebrovascular milieu, using solCD39 (and/or purified CD73) as monotherapy or as an adjunct to low dose thrombolytic therapy. The overarching goal here is to delineate an endogenous cascade which protects vessels against atherothrombosis, and determine whether it may be therapeutically harnessed to ameliorate ischemic stroke outcomes. Relevance to Public Health: This project will explore how two proteins, which exist on cells lining blood vessels, degrade circulating substances which would otherwise promote clotting, inflammation, and atherosclerosis. Harnessing the activity of these proteins might lead to a new way of preventing clot formation and atherosclerosis, and thereby lead to a completely new and perhaps safer treatment for stroke.